1. Field of the Invention
The present invention relates to computer networks, and more particularly to a multi-active fibre channel arbitrated loop adaptive hub.
2. Description of Related Art
Basic RAID storage systems include an array of redundant disks and a controller that enables a server to transparently access the array. A RAID system greatly improves storage system reliability as data can be stored on multiple disks within the array. A RAID system also reduces the cost of storage, when compared to a mirrored storage system solution.
An improved RAID storage system includes dual controllers, each configured to access the same array of disks. The dual controllers improve access to the array because the controllers can simultaneously serve I/O requests from two servers. Moreover, a dual controller (duplex) system can offer even greater reliability than a single controller (simplex) system if each dual controller is configured to handle all I/O requests in the case the other fails. This capability is called “transparent failover.”
Whenever two fibre channel disk array controllers are used in an active—active system configuration, it is vital for that system to continue normal operations even when either one of the two controllers has failed for any reason. In an exemplary conventional dual controller fibre channel arbitrated loop (“FCAL”) system, each of the two disk array controllers has a single FCAL host port. Both of the controllers and the host servers are coupled to a single loop. The controllers can operate in duplex mode (as a redundant pair of controllers) or in simplex mode (as independent controllers).
When configured as a redundant pair, both controllers have access to the same disk drives and both process host server I/O requests. Communication signals between the controllers keep each informed that the other controller is operating normally. If the communication signals are interrupted, the controller that detects the interruption asserts the reset signal to the other controller (holding the failed controller in a hard reset) and starts processing I/O for both controllers. This failover is transparent to the host servers because the surviving controller can respond to multiple target IDs on the host bus. Interruption of the communication signal can result from a controller being removed from the system or the controller experiencing a fault that causes it to lockup when some abnormal operation occurs.
Having the controller host ports and host bus adapters (HBAs) connected on the same loop is a major host port bandwidth limitation of conventional active—active FCAL disk array systems. The overall system bandwidth of a single loop system is approximately one half the bandwidth of a system with two loops, two HBAs and two active—active controllers. In conventional systems both sets of controllers and HBAs are on the same loop to allow for host transparent controller failure recovery.
A need exists for an improved FCAL system that provides the bandwidth and the fault tolerance of a dual active loop controller system, while maintaining the flexibility of a single loop system to overcome the foregoing and other problems of the prior art.